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by crsukumar

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I wish to know why three phase voltage is written as 440V, if one add three phase voltage we gat a sum of 660V, I wish to get a detailed technical answer for the subject please.
many thanks
sukumaran

4 replies

by mjd420nova

Thre phase 440 volt is the common industrial voltage. The 440 volts is measures with a neutral as common. Think of the neutral as the common and you get 440 volts to each phace A B and C. Measure voltage from phase to phase and you would find 277 volts. Most voltage to home is half of the single phase voltage or 220 volts.

by claudiocurcio

In an AC system, the voltage changes in the time with a sin function.

Supouse a vetor rotating at a given frequency, the voltage in a certain point is given by the sin function in that point.

So, if:
Vt: Voltage in time t
Vmax: Max Voltage. You usualy know the efective voltage (Ev) (220V or 110V). The Vmax is computed as Vmax=Ev*SQRT(2) (For 220V it is 311,08 Volts)
f: ferquency
t: time
The voltage in time t is:

Vt = Vmax * sin(2*3,14*f*t)

A three phase system, will have three vectors rotating at a certain frequency, with a phase diference of 120?. because of that the max voltage between each one is not the direct addition of the two voltages.

In a three phase system, the efective voltages between phases (Evp) is:

Evp = Ev * SQRT(3)
381,05 V = 220 V * 1,7332

I hope to have helped you.
Regards,

Claudio

by thechas

Since the dawn of AC power, AC voltages have been expressed as an equivalent to the DC voltage that would generate the same heating power in a resistive load. This is known as the RMS (Root Mean Square) value of the AC voltage.

For single phase AC voltages, the RMS value is the peak voltage times the square root of 2.

As the answer above correctly states, 3 phase power is named such because the 3 legs are 120? out of phase with each other.

This is also why it is critical to connect the phases in the proper sequence to a 3 phase motor.

If you take a close look at what is often referred to as 208 volt 3 phase, you will find that you have 3 individual 120 VAC lines to neutral. When you measure between any 2 phases, you will measure 208 volts.

Now, normal household power is 240 volt single phase. There are 2 in phase legs on each side of the neutral line. The only reason you have 240 volts across both of the 120 volt lines is because both sides are in phase.

Chas

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